The invention relates to bonded abrasive articles or grinding tools made porous by the use of certain agglomerated abrasive grains and to methods for making the agglomerated abrasive grains.
Grinding tools are manufactured in a variety of grades or structures determined by the relative volume percentage of abrasive grain, bond and porosity within a composite abrasive grain matrix. In many grinding operations, grinding tool porosity, particularly porosity of a permeable, or an interconnected nature, improves efficiency of the grinding operation and quality of the workpiece being ground. Porosity inducers, such as bubble alumina and naphthalene, may be added to abrasive composite mixtures to permit pressure molding and handling of a porous uncured abrasive article and to yield an adequate volume percent porosity in the final tool.
Natural porosity arising from packing of the abrasive grains and bond particles during pressure molding is insufficient to achieve a porosity character that is desirable for some grinding operations. Pore inducers have been added to achieve high porosity percentages, however, open channels or interconnected porosity cannot be achieved with the pore inducers known in the art (e.g., hollow ceramic or glass spheres). Some pore induces must be burnt out of the abrasive matrix (e.g., walnut shells and naphthalene), giving rise to various manufacturing difficulties. Further, the densities of pore inducers, bond materials and abrasive grains vary significantly, often causing stratification of the abrasive mix during handling and molding, and, in turn, loss of homogeneity in the three-dimensional structure of the finished abrasive article.
The volume percent of interconnected porosity, or fluid permeability, has been found to be a more significant determinant of grinding performance of abrasive articles than mere volume percent porosity. U.S. Pat. No. 5,738,696 to Wu discloses a method for making bonded abrasives utilizing elongated abrasive grain having an aspect ratio of at least 5:1. The bonded abrasive wheels have a permeable structure containing 55–80%, by volume, of interconnected porosity. The interconnected porosity allows removal of grinding waste (swarf) and passage of cooling fluid within the wheel during grinding. The existence of interconnected porosity is confirmed by measuring the permeability of the wheel to the flow of air under controlled conditions. The filamentary abrasive grains are not agglomerated or otherwise coated with bond prior to assembling the wheel. U.S. Pat. No. 5,738,697 to Wu discloses high permeability grinding wheels having a significant amount of interconnected porosity (40–80%, by volume). These wheels are made from a matrix of fibrous particles having an aspect ratio of at least 5:1. The fibrous particles may be sintered sol gel alumina abrasive grain or ordinary, non-fibrous abrasive grains blended with various fibrous filler materials such as ceramic fiber, polyester fiber and glass fiber and mats and agglomerates constructed with the fiber particles. The filamentary abrasive grains are not agglomerated or otherwise coated with bond prior to assembling the wheel.
Abrasive grain has been agglomerated for various purposes, principal among them to allow use of a smaller particle (grit) size to achieve the same grinding efficiency as a larger abrasive grit size. In many instances abrasive grain has been agglomerated with bond materials to achieve a less porous structure and a denser grinding tool, having more strongly bonded abrasive grains. Agglomerated abrasive grains have been reported to improve grinding efficiency by mechanisms entirely unrelated to the amount or character of the porosity of the abrasive article.
U.S. Pat. No. 2,194,472 to Jackson discloses coated abrasive tools made with agglomerates of a plurality of relatively fine abrasive grain and any of the bonds normally used in coated or bonded abrasive tools. Organic bonds are used to adhere the agglomerates to the backing of the coated abrasives. The agglomerates lend an open-coat face to coated abrasives made with relatively fine grain. The coated abrasives made with the agglomerates in place of individual abrasive grains are characterized as being relatively fast cutting, long-lived and suitable for preparing a fine surface finish quality in the work-piece.
U.S. Pat. No. 2,216,728 to Benner discloses abrasive grain/bond aggregates made from any type of bond. The object of the aggregates is to achieve very dense wheel structures for retaining diamond or CBN grain during grinding operations. If the aggregates are made with a porous structure, then it is for the purpose of allowing the inter-aggregate bond materials to flow into the pores of the aggregates and fully densify the structure during firing. The aggregates allow the use of abrasive grain fines otherwise lost in production.
U.S. Pat. No. 3,048,482 to Hurst discloses shaped abrasive micro-segments of agglomerated abrasive grains and organic bond materials in the form of pyramids or other tapered shapes. The shaped abrasive micro-segments are adhered to a fibrous backing and used to make coated abrasives and to line the surface of thin grinding wheels. The invention is characterized as yielding a longer cutting life, controlled flexibility of the tool, high strength and speed safety, resilient action and highly efficient cutting action relative to tools made without agglomerated abrasive grain micro-segments.
U.S. Pat. No. 3,982,359 to Elbel teaches the formation of resin bond and abrasive grain aggregates having a hardness greater than that of the resin bond used to bond the aggregates within an abrasive tool. Faster grinding rates and longer tool life are achieved in rubber bonded wheels containing the aggregates.
U.S. Pat. No. 4,355,489 to Heyer discloses an abrasive article (wheel, disc, belt, sheet, block and the like) made of a matrix of undulated filaments bonded together at points of manual contact and abrasive agglomerates, having a void volume of about 70–97%. The agglomerates may be made with vitrified or resin bonds and any abrasive grain.
U.S. Pat. No. 4,364,746 to Bitzer discloses abrasive tools comprising different abrasive agglomerates having different strengths. The agglomerates are made from abrasive grain and resin binders, and may contain other materials, such as chopped fibers, for added strength or hardness.
U.S. Pat. No. 4,393,021 to Eisenberg, et al, discloses a method for making abrasive agglomerates from abrasive grain and a resin binder utilizing a sieve web and rolling a paste of the grain and binder through the web to make worm-like extrusions. The extrusions are hardened by heating and then crushed to form agglomerates.
U.S. Pat. No. 4,799,939 to Bloecher teaches erodable agglomerates of abrasive grain, hollow bodies and organic binder and the use of these agglomerates in coated abrasives and bonded abrasives. Higher stock removal, extended life and utility in wet grinding conditions are claimed for abrasive articles comprising the agglomerates. The agglomerates are preferably 150–3,000 microns in largest dimension. To make the agglomerates, the hollow bodies, grain, binder and water are mixed as a slurry, the slurry is solidified by heat or radiation to remove the water, and the solid mixture is crushed in a jaw or roll crusher and screened.
U.S. Pat. No. 5,129,189 to Wetshcer discloses abrasive tools having a resin bond matrix containing conglomerates of abrasive grain and resin and filler material, such as cryolite.
U.S. Pat. No. 5,651,729 to Benguerel teaches a grinding wheel having a core and an abrasive rim made from a resin bond and crushed agglomerates of diamond or CBN abrasive grain with a metal or ceramic bond. The stated benefits of the wheels made with the agglomerates include high chip clearance spaces, high wear resistance, self-sharpening characteristics, high mechanical resistance of the wheel and the ability to directly bond the abrasive rim to the core of the wheel. In one embodiment, used diamond or CBN bonded grinding rims are crushed to a size of 0.2 to 3 mm to form the agglomerates.
U.S. Pat. No. 4,311,489 to Kressner discloses agglomerates of fine (≦200 micron) abrasive grain and cryolite, optionally with a silicate binder, and their use in making coated abrasive tools.
U.S. Pat. No. 4,541,842 to Rostoker discloses coated abrasives and abrasive wheels made with aggregates of abrasive grain and a foam made from a mixture of vitrified bond materials with other raw materials, such as carbon black or carbonates, suitable for foaming during firing of the aggregates. The aggregate “pellets” contain a larger percentage of bond than grain on a volume percentage basis. Pellets used to make abrasive wheels are sintered at 900° C. (to a density of 70 lbs/cu. ft.; 1.134 g/cc) and the vitrified bond used to make the wheel is fired at 880° C. Wheels made with 16 volume % pellets performed in grinding with an efficiency similar to that of comparative wheels made with 46 volume % abrasive grain. The pellets contain open cells within the vitrified bond matrix, with the relative smaller abrasive grains clustered around the perimeter of the open cells. A rotary kiln is mentioned for firing pre-agglomerated green aggregates to later foam and sinter to make the pellets.
U.S. Pat. No. 5,975,988 to Christianson discloses coated abrasive articles include a backing and an organic bonded abrasive layer where the abrasive is present as shaped agglomerates in the shape of a truncated four-sided pyramid or cube. The agglomerates are made from superabrasive grains bonded in an inorganic binder having a coefficient of thermal expansion which is the same or substantially the same as a coefficient of thermal expansion of the abrasive grain.
WO 00/51788 to Stoetzel, et al, discloses abrasive articles have a backing, an organic bond containing hard inorganic particles dispersed within it, and abrasive particle agglomerates bonded to the backing. The abrasive particles in the agglomerates and the hard inorganic particles in the organic bond are essential the same size. Agglomerates may be randomly or precisely shaped and they are made with an organic bond. The hard inorganic particles may be any of a number of abrasive grain particles.
U.S. Pat. No. 6,086,467 to Imai, et al, discloses grinding wheels contain abrasive grain and grain clusters of filler grain having a smaller size than the abrasive grain. Vitrified bond may be used and the filler grain may be chromium oxide. The size of the grain clusters is ⅓ or more of the size of the abrasive grain. Benefits include controlled bond erosion and abrasive grain retention in low force grinding applications utilizing superabrasive grain wherein the superabrasive grain must be diluted to minimize grinding forces. Clusters of filler grain may be formed with wax. No sintering of the clusters is disclosed.
WO 01/04227 A2 to Adefris, et al, discloses an abrasive article comprises a rigid backing and ceramic abrasive composites made of abrasive particles in a porous ceramic matrix. The composites are held to the backing with a metal coating, such an electroplated metal.
None of these prior art developments suggest the manufacture of abrasive articles using porous agglomerated abrasive grain and bond particles to control the percentage and character of porosity and to maintain porosity in the form of permeable, interconnected porosity in bonded abrasive articles. No suggestion is made to use a rotary calciner method to manufacture a variety of abrasive grain agglomerates for use in the abrasive articles. The methods and tools of the invention yield new structures from agglomerated mixtures of existing abrasive grain and bond combinations, and they are sophisticated in permitting the controlled design and manufacture of broad ranges of abrasive article structures having beneficial, bi-modal, interconnected porosity characteristics. Such bi-modal, interconnected porosity enhances abrasive tool performance, particularly in large contact area, precision-grinding operations, such as creepfeed surface grinding, inner diameter grinding and toolroom grinding processes.